Enones ketones

This chapter deals with [2 + 2]cycloadditions of various chromophors to an olefinic double bond with formation of a four-membered ring, with reactions proceeding as well in an intermolecular as in an intramolecular pattern. Due to the variety of the starting materials available (ketones, enones, olefins, imines, thioketones, etc.. . .), due to the diversity of products obtained, and last but not least, due to the fact that cyclobutanes and oxetanes are not accessible by such a simple one-step transformation in a non-photo-chemical reaction, the [2+2]photocycloaddition has become equivalent to the (thermal) Diels-Alder reaction in importance as for ring construction in organic synthesis. 

As we have already seen, delocalization of electrons by conjugation decreases the energy difference between the HOMO and LUMO energy levels, and this leads to a red shift. Alkyl substitution on a conjugated system also leads to a (smaller) red shift, due to the small interaction between the cr-bonded electrons of the alkyl group with the K-bond system. These effects are additive, and the empirical Woodward-Fieser rules were developed to predict the 2max values for dienes (and trienes). Similar sets of rules can be used to predict the A ax values for a,P-unsaturated aldehydes and ketones (enones) and the Amax values for aromatic carbonyl compounds. These rules are summarized in Table 2.4. 

Using the Woodward-Fieser rules, the of dienes and trienes, a,p-unsaturated aldehydes and ketones (enones), and for aromatic carbonyl compounds can be reliably calculated and predicted. 

Finally, it is noteworthy that Lewis base adducts of gallane (I.GalL) reduce cyclic ketones, enones and w-haloketones to the corresponding alcohols in excellent yields254. These reagents show some promise as a new extension of the boron-type reductions of carbonyl compounds. 

Recently, a variety of (3-silylated carboxonium ions have been prepared and characterized by NMR spectroscopy.541 Kira et al.631 used the Corey hydride transfer method, whereas Olah, Prakash, and co-workers applied triphenylmethyl tetrakis (pentafluorophenyl)borate to silylate esters,632 ketones, enones, and carbonates633 [Eq. (3.91)]. The ions thus produced are resonance hybrids of oxocarbenium (327b) and carboxonium (327a) ions with the latter as the major contributors. Calculated (DFT/IGLO) 29 Si NMR chemical shifts agree well with the experimental data. 

It has been demonstrated by Olah et al.420 that a,/3-unsaturated ketones are O-protonated in HF-SbF5 to form hydroxyallylic cations, which were directly observed by NMR spectroscopy. Jacquesy and Coustard have found indirect evidence for diprotonation of a,/3-unsaturated ketones (enones) by trapping the dication with CO.421 The resulting acylium ion centers are then quenched with methanol or benzene. An interesting synthetic method was therefore developed for carboxylation of bicyclic enones in superacid media at atmospheric pressure [Eqs. (5.158) and (5.159)]. 

On the basis of differences in electronegativity, the C - Si bond possesses 12% ionic character. Silicon is the positive end and carbon is the negative end of the dipole. Trimethylftrifluoro-methyl)silane is thus a reagent for introducing the trifluoromethyl group into carbonyl compounds (aldehydes, ketones, enones).24,67 74... 

This heteroannular dienone system causes rings A and B to assume a half-chair conformation. Carbon atoms 1, 3, 4, 5, 6, 8, 9, 10, 11 are all in one plane while carbon atom 2 projects above and carbon atom 7 below the plane. The rest of the molecule is below the plane. Addition of the second unsaturation to the A -3-ketone enone system provides an additional electron-rich unsaturated residue, i.e., it enhances the electron delocalization. The result will be an enlarged contribution of ionic resonance structures such as... 

The Michael reaction of silyl enol ethers with a,P-unsaturated ketones (enones) has generally been performed with the aid of a stoichiometric amount of Lewis acid to afford 1,5-dicarbonyl compounds 80,81). When the reaction was conducted thermally in acetonitrile (82), under high pressure (Si), or by the catalytic use of (Me2N)3S MesSiFj" (84) or Ph3C CI04 (S5), the intermediate adducts were isolable in the form of synthetically valuable silyl enol ethers. 

Some fundamental studies were made earlier. It was found that the hydrodimerization of an O, j6-unsaturated ketone (enone) was at least partially stereospecific, with the d, I dimer formed from the /ranj-enone and mainly the meso dimer formed from the cw-enone. Kanetsuna and Nonaka [50] also examined both the stereoselectivity and stereospecificity of the hydrodimerization in detail. It was indicated that either the reaction mechanism or the stereochemical course is changed by the nature (protic or aprotic) of the reaction media. Particularly, the latter is greatly influenced by the manner of adsorption of an anion radical intermediate on the cathode. It was also postulated to explain the experimental results that the radical anion is adsorbed at the )6-position, where the radical p electron is delocalized and the adsorption is affected by the nature of the media. 

R. Noyori developed a procedure that avoids migration of the double bond during acetalization of a, 3-unsaturated ketones (enones)A Treatment of the enone with the silylated diol l,2-bis[(trimethylsilyl)oxy]ethane in the presence of a catalytic amount of trimethylsilyl trifluoromethanesulfonate (TMSOTf ) affords the acetal without double bond migration (kinetic control). The stability of the Me3Si-0-SiMe3 formed (instead of HjO) drives the reaction to completion. Modifications of Noyori s procedure in which preparation of the silylated diol is circumvented have been reported. ... 

Conjugated ketones (enones) fonn more stable complexes with Lewis acids compared to saturated ketones (alkanones). This leads to preferential functionalization of enones when Eu(fod)3 [52] and Bu2Sn(OTf)T [35] are subjected to chemical transformations (Schemes 2-22 and 2-23). The exposure of the thiolamine (Scheme 2-23) to a mixture of enone and alkanone units in the steroid also shows similar preference for the reaction with the enone [53]. 

Diketene similarly reacts readily with tributyltin oxide in THF at room temperature47 to give the stannyl 3-stannyloxybut-3-enoate which rearranges to the 2-enolate this enolate will then give an ene reaction with an aldehyde,48 or react with an alkyl halide,49 followed by decarboxylation, providing a route to ketones, enones, and ketoaldehydes. Similar reactions with bromomethyl P- or y-lactones result in the formation of P-hydroxy-y-lactones or y-hydroxy-5-lactones. 

In addition to being more selective, dissolved calcium metal functions in a similar way to lithium and sodium metals towards organic functional groups [45]. Tab. 4.2 lists reductions giving the same products by the three dissolved metals. Among these, calcium affords the highest yields for some substrates (entries 1-3). The compounds in Tab. 4.2 include an aldehyde, indole [46], aryl ketone, enone, naphthalene [47], pyridine N-oxide [48], benzyl alcohol, styrene, and buckminster-fullerene. 

In order to facilitate recycling of the multiple TsDPEN-functionalized dendrimer catalysts, the same group recently reported the synthesis of a novel form of hybrid dendrimer ligands by coupling polyether dendrons with peripherally TsDPEN-functionahzed Newkome-type poly(ether-amide) dendrimer (Figure 4.28) [90]. The solubility of these hybrid dendrimers was found to be affected by the generation of the polyether dendron. The ruthenium complexes produced were applied in the asymmetric transfer hydrogenation of ketones, enones, imines and activated... 

The additions of trialkylstannyl anions to aldehydes, ketones, enones, and alkynes also occur to afford tetraorganostannanes with various functional groups. a-Alkoxyalkylstannanes prepared by the reactions of trialkylstannyllithium with carbonyl compounds followed by the protection of their hydroxyl groups are the useful precursors of a-alkoxyalkyllithiums (eq (137)) [132]. 

Dichloroindium hydride, generated by transmetallation between tributyltin hydride and indium trichloride, predominantly reduces unsaturated ketones (enones) with 1,4 selectivity in the presence of aldehydes. Under anhydrous conditions, the successive aldol reaction between the resulting enolates and the remaining aldehydes proceeds with high anti-selectivity. The stereochemistry, however, is reversed to be syn-sclccti vc by the use of water and methanol as an additive and solvent, respectively [12 b]. 

Homoallylic alcohols are formed by combining allyl halides with aldehydes and ketones in the presence of low-valency chromium ions. Aldehydes react faster than ketones enones react very slowly while many other functional groups are inert. The y-carbon atom of the allyl group bonds to the carbonyl carbon atom (Scheme... 

Since oxidation and reduction reactions can provide many organic compounds with reactive functional groups such as aldehydes, ketones, enones, amines, alcohol, allylic alcohols, and so on, further transformations can easily be added to give a domino process. Depending upon the position of the oxidation or reduction reaction in the domino process, this chapter is divided into three classes first, the domino reaction is initiated by an oxidation or reduction reaction second, the domino reaction has the oxidation or reduction step in the middle and third, the domino reaction is terminated by an oxidation or reduction reaction. Most of the oxidation and reduction reactions come under the category of anionic domino process, as they provide nucleophilic or electrophihc functionalities and only very few oxidation and reduction reactions proceed with cationic domino process. 

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